1. Field of the Invention
The present invention relates generally to telecommunications, and more particularly to a system and method for removing unwanted direct current (DC) offsets from baseband signals for mobile station modems (MSMs).
2. Background Art
Conventional methods of down converting a Radio Frequency (RF) signal to baseband require two conversion steps. The RF signal is first down converted to an intermediate frequency (IF) signal. Then, the IF signal is down converted to a baseband signal. In a mobile telecommunication environment, this requires a radio frequency receiver (RFR) chip, an intermediate frequency receiver (IFR) chip, a baseband receiver chip, and other associated surrounding chips, all of which are expensive for mobile phone manufacturers.
A direct conversion enables the direct conversion of RF signals to baseband signals in a single step. Thus, direct conversion eliminates the need for the RF to IF conversion step, and thus, the IFR chip.
One of the problems associated with direct conversion is that it results in very high direct current (DC) offset levels. These unwanted DC offsets include static DC levels as well as time varying DC levels. The sources of static and time-varying DC offsets include circuit mismatch, LO self-mixing, and interferer self-mixing, each of which may vary with gain setting, frequency, fading, and temperature. If such DC offsets are not cancelled, they degrade signal quality, limit dynamic range through saturation, and increase power consumption.
What is needed is a system and method that cancels DC offsets for direct conversion architectures. What is also needed is a system and method that compensates for static DC levels and time varying DC levels for direct conversion architectures. What is further needed is a system and method that acquires and cancels DC offsets in a fast and efficient manner for direct conversion architectures.